It has been known for some time that position-finding can be carried out on the basis of radio links, for example within the satellite-assisted GPS (Global Positioning System) system. The European satellite navigation system Galileo, and also positioning methods based on terrestrial radio sources, afford further applications. Such position-finding methods and appropriate appliances allow the user to find his position by measuring the distance to a particular number of wireless signal sources such as satellites or base stations. By way of example, each GPS and Galileo satellite transmits unique digital sequences, which include a time identifier and the satellite position. The signals are usually coded using long spread codes. The spread codes for the individual satellites are virtually orthogonal with respect to one another, so that the signals can be distinguished from one another in the receiver. By way of example, the spread codes for the various GPS and Galileo satellites are synchronized to one another using high-precision atomic clocks installed in the satellites.
The receiver evaluates the relative delays between the signal transmission from various radio sources (GPS satellites, Galileo satellites or terrestrial transmitters) and ascertains delay time offsets therefrom. Together with the data about the position and the time reference of the various radio sources, the delay time offsets can be used to locate the receiver exactly. To this end, the receiver calculates the “pseudo-ranges”, which represent the distance to each radio source. Navigation software can then calculate the user position on the basis of the pseudo-range to each radio source and the position of the radio sources by solving a set of non-linear equations.
Many receivers customary today in position-finding systems are based on the practice of despreading the samples of the received spread-coded position-finding signals at first and then subjecting them to coherent and to non-coherent integration. The resultant statistical values are supplied to a detector, for example a Neyman-Pearson detector, which maximizes the probability of identifying the position-finding signals according to the desired requirements.
The detector compares the statistical values supplied to it with a threshold value. If a statistical value is greater than the threshold value, it is assumed that a position-finding signal has been received. In the opposite case, the received signal is not classified as a position-finding signal. This is intended to prevent signals which are not position-finding signals from being used for position-finding. In addition, this method also prevents position-finding signals with too small a reception amplitude from being used for position-finding.
One problem is that the individual position-finding signals do not always reach the receiver along a direct line-of-sight (LOS) path, but rather are often attenuated by a wide variety of obstacles. These obstacles include the walls and ceilings of buildings, coated windows, bodywork of motor vehicles, sources of shade and treetops. Since the various satellites in satellite navigation systems are distributed as far as possible from one another in prescribed arrangements, the various position-finding signals emitted by the individual satellites reach the receiver from totally different directions. The position-finding signals reaching the receiver on various transmission paths are therefore attenuated in different ways. By way of example, while position-finding signals emitted from one satellite have to pass through a wall, which attenuates them by 25 dB, in order to reach the receiver, the position-finding signals coming from another satellite may reach the receiver via a line-of-sight path. This may result in position-finding signals no longer being identified as such by the detector on account of their attenuation or in the receiver considering signals which are not position-finding signals to be position-finding signals.